Electrically conductive composites consist of conductive particles dispersed in an organic matrix. The conduction threshold, or percolation threshold (a transition between insulator and conductor), is reached when the conductive particles form a network of connected conducting pathways over the entire volume of the composite.
The conductive particles may be metallic particles, which have the advantage of a high electrical conductivity. However, they have the drawback of possessing a high density and of being sensitive to the chemical environment of the cell. Non-metallic particles are particularly useful because of their low density and their chemical resistance. The non-metallic conductive fillers used most often are carbon-based pulverulent products, such as graphite or carbon black powders and carbon fibres.
Depending on the particle morphology (aspect ratio and specific surface area), the percolation threshold is reached for filler fractions of a few vol % in the case of fibres and from 20 to 30 vol % in the case of spheres. Typically, these fillers can be used to obtain conductivities within the volume of the material of the order of 10−5 to 10−1 S/cm. Thus, it should be pointed out that the conductivity of the composites is very much lower than that of the fillers used (of the order of 1000 S/cm in the case of graphite), although the filler fractions are above the percolation threshold. This effect is explained by the large contact resistances between adjacent particles. These resistances are due, on the one hand, to the low contact surface area between two particles (constriction resistance) and, on the other hand, to the formation of an insulating film on the surface of the fillers when they are dispersed in the organic binder (tunnel resistance).
The constriction resistance is defined by the equation Rcr=ri/d, where ri represents the resistivity of the filler and d the diameter of the interparticle contact surface. The coverage area of the fillers is controlled by their geometry and their viscoelastic properties, that is to say their ability to deform under stress.
The tunnel resistance is associated with any insulating film able to cover the surface of the particles. It may be due to absorbed surfactants or more simply to the organic matrix which encapsulates the fillers once they have been dispersed therein. In this configuration, the mechanism of conduction between conductive particles is no longer ohmic, but takes place via electron jumps between isolated particles. Because of the low electron transport properties of polymers, the local electric field between conductive particles, which is necessary for the electric current to be able to flow through the entire conductive clusters formed by the fillers, must be very high. In practice, the local electric field is never high enough to allow an electron to jump between each particle which is connected but insulated by a polymer film. Only a small portion of the conducting pathways is stressed and actually participates in the flow of the current. The macroscopic conductivity is greatly reduced. The tunnel resistance is defined by the equation Rt=rt/a, where rt represents the tunnel resistivity, which depends on the thickness of the film and on the electrical properties of the insulating organic matrix, and where a is the contact area.
The resistance at the particle—particle interface is the sum of the constriction and tunnel resistances. In the great majority of cases, the tunnel resistance governs the macroscopic conductivity of heterogeneous media. This is because the tunnel resistance goes from 10−8 to 103 □·cm when the thickness of the insulating film covering the contacting conductive particles varies from 0.5 to 12 nm. This insulation thickness of a few nanometres conventionally corresponds to the layer of polymer matrix adsorbed on the surface of the fillers during dispersion.
The abovementioned general principles suggest that to produce highly conductive composites requires the processing, by conventional polymer conversion techniques, of materials which are highly filled with conductive elements so as, on the one hand, to increase the number of contacts between conductive particles and, on the other hand, to increase the contact areas between adjacent elements.
These principles, used within the context of developing elements for a fuel cell, have resulted in the following most significant patents being filed:
U.S. Pat. No. 6,106,263 claims a process for producing moulded bodies by extruding plastics filled to more than 50 vol % (preferably 65 to 90 vol %) with conductive elements (lamellar or non-lamellar graphite, conductive fibres, etc). In this case, mixing is carried out in a mixer and then the mixture, after having been crushed and ground, is extruded in the form of sheet or tube. The patent describes the processing operations and the specific and inventive operation of the extruder used. The formulations are produced from any type of commercially available conductive fillers.
Patent applications have been filed for processes consisting in thermally compressing mixtures of graphite (lamellar or non-lamellar) powder in thermoplastics. The main points claimed are the production of sufficiently conductive mouldable materials.
U.S. Pat. No. 5,558,955 claims the production of a stack for a DMFC fuel cell, based on a conductive composite obtained by a thermally compressed mixture of lamellar graphite and a fluororesin (PTFE). The materials produced by this process are non-porous and directly compression-mouldable. The structure of the conductive material and the nature of the fillers are not necessarily optimized for controlling the gas impermeability and the cooling of the cell (deposition of a surface barrier material in order to seal the assembly).
Application US 2002004156 discloses a process for manufacturing separator plates for a fuel cell, based on a thermosetting binder (phenolic/epoxy resin) filled with graphite (lamellar or non-lamellar). The mixture is thermally compressed in a mould to the geometry of the separator plates to be produced. The porosity, and therefore to a first approximation the gas impermeability, is optimized by promoting the removal of the water and gases formed during crosslinking. However, an insulating resin layer covers the surface of the plates and must be removed by stripping.
U.S. Pat. No. 5,942,347 discloses a process for manufacturing, by thermal compression, bipolar separator plates containing from 50 to 95% of conductive filler in various thermoplastics. The problem of gas impermeability is circumvented by the addition of a hydrophilic agent promoting the migration of water in the pores of the material. This configuration promotes above all the removal of the water produced in the cell and allows the cell to be cooled.
U.S. Pat. No. 6,436,567 discloses a process for manufacturing separator plates for fuel cells, having a high conductivity, a low gas permeability and high mechanical properties. The bipolar plate material consists of a compound based on a polymer and on graphite powder, the aspect ratio of which is from 4 to 60 (preferably 10 to 30). This filler morphology allows the gas impermeability and the electrical conductivity to be improved. However, the structure of the filler is not described.
The prior art in the field of electrically conductive filled materials for bipolar plates has therefore essentially described materials based on graphite, the morphology and the structure of which are not clearly explained.
Moreover, in the field of materials for bipolar plates based on polyvinylidene fluoride (PVDF) or more generally on fluoropolymers, the following patent applications shall be mentioned:
Patent DE 353 8732 discloses an electrode made from a paste which may possibly be extended and consists of 70 to 80% by weight of carbon powder having a granule size of 30 to 300 μm and of 10 to 20% by weight of a PVDF solution containing 4 to 8% PVDF in DMF (dimethylformamide) and of at least 5% by weight of PTFE (polytetrafluoroethylene) powder having a granule size of 10 to 100 μm. The paste is spread out over an aluminium substrate and then dried using an infrared lamp for 1/2  h to 4 h. This electrode based on PVDF and carbon is permeable to gases and to liquids.
Patent Application JP 08031231 A discloses a formulation based on spherical graphite, on a thermoset or thermoplastic and on carbon black of the conducive Ketjenblack type. The material shows good mechanical strength and can be used for moulding and calendering. This material can be used in the fuel cell field.
Patent Application JP 04013287 A discloses a carbon plate which is porous in three dimensions with a porosity level of 60% to 80%.
Patent Application JP 52122276 A discloses an electrode prepared by depositing pyrolysed anisotropic carbon onto a porous textile, the carbon itself being coated with an aqueous dispersion of TEFLON® (PTFE), and the whole assembly is dried to form a hydrophobic porous layer.
U.S. Pat. No. 6,248,467 discloses a bipolar plate for use in the fuel cell field, which is obtained by moulding a vinyl ester resin and a graphite powder, allowing a conductivity of at least 10 S/cm to be obtained. These plates may contain from 20% to 95% graphite and from 0% to 5% carbon black, together with cotton fibres. The use of fluoro products to improve the demoulding and the hydrophobicity are also disclosed therein.
U.S. Pat. No. 5,268,239 discloses the preparation of a separator plate. This graphite-based plate is a mixture containing from 25 to 75% by weight of graphite and from 25 to 75% by weight of phenolic resin. Next, this plate is pyrolysed between 800 and 1 000° C. and then graphitized between 2 300° C. and 3 000° C. This patent also describes the application of a fluoropolymer film in order to prevent migration of the electrolyte.
Patent Application WO 2000/24075 discloses the preparation of a substrate that can be used for membrane preparation, this substrate comprising a porous fibre matrix, characterized in that the fibres are adhesively bonded to the silica and a fluoropolymer. It also describes the process, with firstly the dispersion of the fibres in water and then secondly the deposition of this dispersion in order to form a network. The network of fibres is then dried and compacted. An aqueous fluoropolymer dispersion can be introduced before or after this drying and compacting step.
U.S. Pat. No. 4,163,811 discloses a process for preparing an electrode for fuel cells, characterized by the following steps: (i) firstly, the formation of an aqueous suspension of catalyst particles, with addition of a cationic surfactant, then (ii) the formation of a second colloidal aqueous suspension of a hydrophobic polymer and (iii) the mixing of the two suspensions in order to form a uniform aqueous suspension of catalyst particles and hydrophobic polymer particles. This suspension is then deposited on a conductive support and heated so as to sinter the layer of catalyst and polymer.
U.S. Pat. No. 4,177,159 discloses a process for preparing a dry finely divided powder characterized in that it consists of particles having a maximum size of approximately 5 μm. This powder comprises precatalysed carbon and a hydrophobic fluorocarbon polymer, for example PTFE. This powder is obtained by flocculating a cosuspension of the precatalysed carbon particles and the polymer particles.
U.S. Pat. No. 6,455,109 and US 2003096154 discloses a method for producing an electrode for fuel cells and a catalytic powder prepared by mixing a fine carbon powder supporting a catalytic metal with a colloidal dispersion of a polymer. The suspension thus obtained is dried.
U.S. Pat. No. 5,846,670 discloses the preparation of a gas diffusion electrode for an electrochemical cell. This electrode is prepared using a carbon black powder dispersed in an organic solvent in the presence of soluble polyethylene. The dispersion is then dried, allowing the polyethylene to cover the surface of the black. This polyethylene is then fluorinated. Next, this hydrophobic carbon black powder is mixed with an acetylene carbon black supporting a catalyst metal and PTFE in order to form aggregates. Next, these aggregates are pressed at 20 kg/cm2 and sintered at 340° C. for 20 minutes.
U.S. Pat. No. 6,103,077, U.S. Pat. No. 6,368,476, and U.S. Pat. No. 6,444,601 describe a method of preparing a gas-permeable electrode by making a dispersion of carbon black particles or of carbon black particles supporting a catalyst using a high-shear apparatus in order to homogenize it, such as a microfluidizer, then by adding a binder to the dispersion obtained followed by a stabilizer. Next, this mixture is deposited on an electrically conducting fabric and then dried and sintered at 300-400° C.
U.S. Pat. No. 6,180,275 discloses a mouldable composition used for preparing current collector plates by compression moulding or injection moulding. This composition comprises a non-fluorinated polymer binder; the polymers that can be used include polyphenylene sulphides, modified polyphenylene ethers, liquid-crystal polymers, polyamides, polyimides, polyesters, phenolic resins, epoxy resins and vinyl esters. The conducting particles include more particularly carbon particles. These carbon particles are present in an amount of at least 45% by weight.
Makoto Ushida, in J. Electrochem. Soc., Vol. 142, No. 12, December 1995, studied the preparation of an MEA (membrane and electrode assembly), based on the formation of a colloid for optimizing the formation of a network in the layer of catalyst and for simplifying the manufacture of the MEA. The preparation, by producing, for example, a mixture of perfluorosulfonate ionomer (PFSI) dissolved in ethanol, receives an addition of butyl acetate (a poor solvent) in order to form a colloidal solution. Next, a platinum-supporting carbon is mixed with a carbon coated with PTFE. This PTFE-coated carbon is prepared by mixing a carbon suspension and a PTFE suspension together with a surfactant, and the surfactant is then removed during a treatment in air at 290° C. The mixture of the two powders, Pt/C and C/PTFE, is added to the PFSI colloidal solution, which gives rise to crosslinking of the PFSI chains adsorbed by the carbon, this crosslinking being promoted by an ultrasonic treatment. This colloidal suspension is then spread out onto a carbon paper which is pressed at 130° C. and 7.5 MPa for 1 minute.
In Journal of applied electro chemistry 28 (1998) pp 277-282 Fischer studied the preparation of an MEA by spraying a mixture of a slurry (suspension) of a catalyst metal, of a Nafion® (fluoroacrylate) solution in water and of glycerol onto a heated membrane based on Nafion 117®. The solvents are then evaporated by heating to 150°C.
U.S. Pat. No. 4,214,969 discloses a bipolar plate for fuel cells, which consists of graphite and a fluoropolymer in a ratio of 2.5:1 to 16:1. These bipolar plates have a volume resistivity of 4×10−3 Ω·in. This graphite/fluoropolymer mixture is dry-blended in a blender for 25 minutes and then introduced into a hot compression mould.
Patent Application GB 22 20666 discloses a cospraying method for preparing carbon black particles coated very uniformly with synthetic latex particles. There is no mention of a fluoropolymer in the description or in the examples.
The prior art has essentially disclosed the production of bipolar plates by batch processes using solvents or processes involving only coarse mixtures of the various products used to produce the plates. The prior art disclosing coatomization does not disclose fluoropolymers. Nor does the literature describe graphite having a morphology allowing the direct production, by moulding, of composite plates having a high impermeability, a high surface conductivity and a high thermal conductivity.
The Applicant has now found a highly homogeneous microcomposite powder based on a fluoropolymer and on graphite flakes having a morphology consisting of superimposed parallel graphite lamellae having a high interparticle contact area. This powder can be processed by the techniques normally used for thermoplastics and makes it possible to obtain, directly after moulding, bipolar plates having a high surface conductivity. The gas impermeability levels depend on the conditions under which the plate is processed. This microcomposite powder may be produced by coatomizing an aqueous dispersion comprising a fluoropolymer and these graphite flakes.
The process for manufacturing the microcomposite powder does not involve any solvent other than water. The objects thus manufactured are useful in fuel cells.
The advantages and further features of the present invention will be explained in the detailed description of the invention which follows.